Optimization of Acetylene Black Conductive Additive and PVDF Composition for High-Power Rechargeable Lithium-Ion Cells G. Liu, a, * ,z H. Zheng, a,c A. S. Simens, b,d A. M. Minor, b X. Song, a and V. S. Battaglia a, * a Lawrence Berkeley National Laboratory, Environmental Energy Technologies Division and b National Center for Electron Microscopy, Material Science Division, Berkeley, California 94720, USA Fundamental electrochemical methods were applied to study the effect of the acetylene black AB and the polyvinylidene difluoride PVDF polymer binder on the performance of high-power designed rechargeable lithium-ion cells. A systematic study of the AB/PVDF long-range electronic conductivity at different weight ratios is performed using four-probe direct current tests, and the results are reported. There is a wide range of AB/PVDF ratios that satisfy the long-range electronic conductivity require- ment of the lithium-ion cathode electrode; however, a significant cell power performance improvement is observed at small AB/PVDF composition ratios that are far from the long-range conductivity optimum of 1 to 1.25. Electrochemical impedance spectroscopy EIS tests indicate that the interfacial impedance decreases significantly with an increase in binder content. The hybrid power pulse characterization results agree with the EIS tests and also show improvement for cells with a high PVDF content. The AB to PVDF composition plays a significant role in the interfacial resistance. We believe the higher binder contents lead to a more cohesive conductive carbon particle network that results in better overall all local electronic conductivity on the active material surface and, hence, reduced charge-transfer resistance © 2007 The Electrochemical Society. DOI: 10.1149/1.2792293 All rights reserved. Manuscript submitted June 17, 2007; revised manuscript received August 21, 2007. Available electronically October 22, 2007. Lithium-ion rechargeable batteries are a prime candidate for electric vehicle EV and hybrid electric vehicle HEV applications due to their high-energy density and light weight. These applica- tions, especially HEV, require low internal resistance for superb high-rate charge and discharge performance. The lithium-ion cell electrode is composed of active materials, conductive additives, and a polymer binder to combine the particles into an integrated elec- trode system. The cathode active material is made from metal oxide materials, which have very low intrinsic conductivity ranging from 10 -3 S/cm for LiCoO 2 to 10 -9 S/cm for LiFePO 4 at ambient condition. 1-4 The active material primary particles are sintered into micrometer-size particles and mixed with highly conductive carbon additives to improve the particle conductivity. In this study, LiNi 0.8 Co 0.15 Al 0.05 O 2 is used as active material, acetylene black AB is used as conductive additive, and polyvinylidene difluoride PVDF is used as polymer binder. The focus here is the optimiza- tion of the composition of the porous composite electrodes to im- prove the lithium-ion cell performance. A unique approach is taken to study the lithium-ion battery cathode electrode as a polymer composite. A simple cathode is a three-component composite including a polymer binder and two discreet sized particles: the nanosize AB and the microsize LiNi 0.8 Co 0.15 Al 0.05 O 2 . The specific surface area of the AB is at least ten times larger than that of the LiNi 0.8 Co 0.15 Al 0.05 O 2 material. In a polymer composite system, the surface area dominants the mixing process such that most of the polymer binder in a composite will associate with the smaller size conductive additive, even when the AB is far from being the weight-dominant component. 5-7 In this respect, we view the electrode system as a LiNi 0.8 Co 0.15 Al 0.05 O 2 cathode material being bond together by an AB/PVDF composite. The electronic properties of the AB/PVDF composites naturally af- fect the performance of the electrode. 8 In this report, a set of experiments were designed to evaluate the contribution of the long-range electronic conductivity of AB/PVDF composites to the overall electrode performance. The long-range electronic conductivities of the AB/PVDF composites and the AB/ PVDF/active material composites were measured via four-point probe direct-current dc method. 9 The composite films were cast on glass for the four-point probe dc measurements. Lithium-ion cells of very similar capacity and configuration but with various the AB/ PVDF ratios were constructed, and their power performance evalu- ated with variable rate cycling, hybrid pulse power characterization HPPC experiments, and electrochemical impedance spectroscopy EIS. Most of the previous work on electrode compositions treats the conductive additive and binder as two independent variables: one present for providing electronic conductivity and the other present to hold the electrode components together. 10 Although AB does pro- vide electronic conductivity in the cathode electrode, long-range electron pathways cannot be formed without the participation of the binders. Therefore, AB and PVDF are the integrated parts of the electrode rather than two independent components. 11-15 We report the electronic properties of the AB/PVDF composites and their ef- fects to the electrode performance. Experimental Materials.— Battery-grade AB with an average particle size of 40 nm and a material density of 1.95 g/cm 3 was acquired from Denka Singapore Private Limited. PVDF no. 1100 binder with a material density of 1.78 g/cm was supplied by Kureha, Japan. An- hydrous N-methylpyrrolidone NMP was purchased from Aldrich Chemical Company. The AB/PVDF mixtures were made by dissolv- ing 5 g of PVDF in 95 g of anhydrous NMP. A given amount of AB was dispersed in the PVDF polymer solution to meet the desired ratio. To ensure the thorough mixing of the AB nanoparticles into the polymer solution, sonification was used. A Branson 450 sonica- tor equipped with a solid horn was used. The sonication power was set at 70%. A continuous sequence of 10 s pulses followed by 30 s rests was used. The sonic dispersion process took 30 min. The slurry properties for all AB/PVDF in NMP were constant after 20 min of sonification. Slurries with active cathode material were made by adding the targeted amount of LiNi 0.8 Co 0.15 Al 0.05 O 2 active material to the freshly premixed AB/PVDF/NMP slurry. The cathode mixture was homogenized using Polytron PT10-3S homog- enizer at 3000 rpm for 15 min until a viscous slurry was acquired. LiNi 0.8 Co 0.15 Al 0.05 O 2 , with a mean particle size of 10 m and lattice density of 4.73 g/cm 3 , was a gift from Toda, Japan. The manufacturer-suggested specific capacity is 173 mAh/g when cycled between 3 and 4.1 V. * Electrochemical Society Active Member. c Permanent address: Henan Normal University, Henan Provence, China. d Present address: Material Science and Engineering Department, Stanford Univer- sity, Stanford, California 94305, USA. z E-mail: Gliu@lbl.gov Journal of The Electrochemical Society, 154 12 A1129-A1134 2007 0013-4651/2007/15412/A1129/6/$20.00 © The Electrochemical Society A1129